1. Field of the Invention
The present invention relates to a toner and a developer for developing an electrostatic latent image used in such a process as an electrophotographic process and an electrostatic recording process.
2. Description of the Related Art
The image forming process by visualization of image information through an electrostatic latent image by the electrophotographic system (hereinafter, sometimes referred to as an electrostatic image) has been used in various fields of art. A technique for providing higher image quality is being demanded according to the digitalization and progress in image processing techniques in recent years.
In reaction to the demand of higher image quality, a toner for developing an electrostatic image has been improved to have a smaller particle diameter and a uniform particle size distribution. However, there is a limitation in reducing the particle diameter by the conventional kneading and pulverizing process, and the uniformization in particle size distribution is still insufficient for obtaining higher image quality even by using a classification step.
Such characteristics are being also strongly demanded in the toner for developing an electrostatic image, as electric power saving, energy saving, low cost and prolonged service life, in view of environmental protection. In order to attain the demand, for example, from the standpoint of the fixing technique, the prolonged service life is attained by oilless fixing, and the energy saving and low cost are attained by fixing at a low temperature and a high speed. As a practical method for attaining the techniques, such a method is being generally practiced that a releasing agent, such as wax, is contained in a toner to impart a releasing effect to the toner itself.
However, the structure and the addition amount of the releasing agent are difficult to control by the conventional kneading and pulverizing process, as similar to the case of obtaining higher image quality, and thus it is the current situation that the demand cannot be attained by these measures.
As a method for intentionally controlling the structure of a toner for developing an electrostatic image, JP-A-63-282752 and JP-A-6-250439 propose processes for producing a toner by the emulsion polymerization aggregation process. In these processes, a resin dispersion liquid is produced by the emulsion polymerization method, and separately, a colorant dispersion liquid is produced by dispersing a colorant in a solvent. The dispersion liquids are then mixed to form aggregated bodies having a diameter corresponding to the toner diameter, which are then fused and integrated by heating. In the process, the shape of the toner can be controlled to some extent to improve the charging property and the durability. However, the toner has a substantially uniform interior structure, and thus there are problems remaining in the releasing property of a fixing sheet upon fixing, and the fixing property at a low temperature and a high speed.
JP-A-5-61239 proposes a toner for oilless fixing containing a large amount a releasing agent component encompassed in the toner. However, the releasing property can be improved to some extent owing to the large amount of the releasing agent added, but the releasing agent cannot be exuded stably and uniformly due to the compatibility between the binder component and the releasing agent, and thus stable releasing property cannot be obtained. Furthermore, the compatibility of the releasing agent with the binder component lowers the glass transition temperature of the toner, which deteriorates the storage stability and the document offset property. The dispersing properties of the materials inside the toner exert great influence not only on the characteristics of the fixed image, such as the adhesion property to paper, the releasability from the fixing roll, the flexural resistance after fixing and the gloss, but also on total fixing property of the toner, such as the OHP transparency.
A method for improving the fixing property by defining the components and the melting heat amount is proposed. As a method for improving the fixing property by defining the carbon number of the releasing agent, for example, JP-A-8-152735 proposes a toner having good fixing property and causing less filming by defining the carbon number of the releasing agent and the proportion of a linear hydrocarbon. However, the fixing capability at high speed cannot be sufficiently obtained by the carbon number defined in the toner, and the tendency is conspicuous in oilless fixing.
As a method for improving the fixing property by defining the melting heat amount of the releasing agent, for example, JP-A-2000-3077 and JP-A-6-67504 propose such a toner that the wax used as the releasing agent of the toner is defined in melting heat amount to improve the offset resistance. However, the wax cannot be stably exuded upon fixing due to the compatibility between the binder component of the toner and the wax, and improvement cannot be necessarily obtained in hot offset. The tendency is conspicuous in oilless fixing. Furthermore, no sufficient effect is obtained on deterioration in storage stability of the toner and the resulting image due to the reduction in glass transition temperature of the toner by the wax.
As a method for improving the fixing property by defining the components of the releasing agent, for example, JP-A-2000-321815 proposes such a toner that the proportion of a normal paraffin and the DSC endothermic curve of the toner are defined to improve the fixing property, the storage stability, the flowability and the durability. However, the method can improve the plasticizing effect to the binder resin of the toner to improve the fixing property, the storage stability, the flowability and the durability to some extent, but the improvements in the method are not necessarily satisfactory.